I received the Dragon empires Gazetteer today and I'm looking it over. Shenmen would be he perfect location for a Manifest-like area in the world. What I would do is house-rule the whole land as a manifest zone, making the city subterranean and overrun by nasty things, with ettercaps, evil fey, wayangs and undead being the primary races. Maybe not have a Manifest city, but do keep the uundercity.

Replace references to the Varlin Mountains to the Gossamer Mountains.

Thematically this all makes sense as Shenmen is the only area in Golarion that has the most ghosts. DMs running with this idea should make it harder for ghosts to move on the the true afterlife. Ghost Eaters should be encountered frequently as they are a huge hazard to to ghosts.